docs: def _split_span(self, span, index, length=0):
          """Split a span into two or three separate spans at certain indices."""
          offset = span[1] + index if index < 0 else span[0] + index
          # log.debug([(span[0], offset), (offset, offset + length), (offset + length, span[1])])
          return [(span[0], offset), (offset, offset + length), (offset + length, span[1])]